Cover - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Nov. 20, 2023	Mar. 31, 2023
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Sep. 30, 2023
Current Fiscal Year End Date	--09-30
Document Transition Report	false
Securities Act File Number	814-00794
Entity Registrant Name	GOLUB CAPITAL BDC, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-2326940
Entity Address, Address Line One	200 Park Avenue
Entity Address, Address Line Two	25th Floor
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10166
City Area Code	212
Local Phone Number	750-6060
Title of 12(b) Security	Common Stock, par value $0.001 per share
Trading Symbol	GBDC
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Auditor Firm ID	42
Auditor Name	Ernst & Young LLP
Auditor Location	Chicago
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 2,245
Entity Common Stock, Shares Outstanding		169,594,742
Documents Incorporated by Reference	Portions of the registrant’s proxy statement to be filed with the Securities and Exchange Commission pursuant to Regulation 14A in connection with the registrant’s 2024 Annual Meeting of Stockholders, which will be filed subsequent to the date hereof, are incorporated by reference into Part III of this Form 10-K. Such proxy statement will be filed with the Securities and Exchange Commission not later than 120 days following the end of the registrant’s fiscal year ended September 30, 2023.
Amendment Flag	false
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
Entity Central Index Key	0001476765